1. Field of the Invention
The present invention relates to a magnetic head drive circuit for applying a current for recording an information signal to a magnetic field generating coil provided for a magnetic head. Furthermore, the present invention relates to a magneto-optical recording device for generating a magnetic field whose direction and intensity can be switched according to the information signal on the magnetic head by applying a current to the magnetic field generating coil provided for the magnetic head from the magnetic head drive circuit, applying the magnetic field to a magneto-optical recording medium, and simultaneously emitting an optical beam for recording by an optical head such that the beam can converge at a portion to which the magnetic field is applied, thereby recording the information signal.
2. Related Background Art
Conventionally, a magneto-optical recording device forms a magnetized area having a variable magnetization state on a magneto-optical recording medium. The magneto-optical recording device records an information signal by a magnetic head applying a magnetic field with its direction and intensity switched according to the information signal to a magneto-optical recording medium, and by an optical head emitting an optical beam for recording such that the beam can focus on a potion to which the magnetic field is applied. Patent Application Laid-Open Gazette 63-94406 shows an example of a magnetic head drive circuit, used in the magneto-optical recording device, for applying a current for recording an information signal to a magnetic field generating coil provided for a magnetic head.
FIG. 4 shows the configuration of the magnetic head drive circuit described in the above mentioned gazette. Reference numeral 30 denotes a magnetic field generating coil provided for the magnetic head. Reference numerals 31a and 31b denote switch elements. Reference numerals 32a and 32b denote auxiliary coils. Reference numerals 33a and 33b denote direct current sources. When an information signal is recorded, the switch elements 31a and 31b are alternately turned on and off according to the information signal. When the switch element 31a is ON and the switch element 31b is OFF, an electric current is applied to the magnetic field generating coil 30 and the switch element 31a from the direct current source 33a through the auxiliary coil 32a. Simultaneously, the current is applied from the direct current source 33b to the switch element 31a through the auxiliary coil 32b. 
Alternatively, when the switch element 31a is OFF and the switch element 31b is ON, an electric current is applied to the magnetic field generating coil 30 and the switch element 31b from the direct current source 33b through the auxiliary coil 32b. Simultaneously, the current is applied from the direct current source 33a to the switch element 31b through the auxiliary coil 32a. The inductance of the auxiliary coils 32a and 32b is larger than that of the magnetic field generating coil 30, and a current Ia of the auxiliary coil 32a and a current Ib of the auxiliary coil 32b can be maintained at a substantially constant value Is during the process of recording an information signal regardless of the status of the switch elements 31a and 31b. For example, the current Is is about 0.15A. However, the current Ib of the auxiliary coil 32b decreases by approximately xcex94I when the switch element 31a is switched from ON to OFF, and the current Ia of the auxiliary coil 32a decreases by approximately xcex94I when the switch element 31b is switched from ON to OFF. The reduction xcex94I of the electric current is small as compared with Is.
Thus, by alternately switching ON and OFF the switch elements 31a and 31b, an electric current through the auxiliary coil 32a and an electric current through the auxiliary coil 32b are alternately applied to the magnetic field generating coil 30. The direction of the current applied to the magnetic field generating coil 30 is alternately switched according to an information signal, and a current with the amplitude of approximately xc2x1Is is applied. In addition, a voltage VQa at a connection point Qa between the auxiliary coil 32b and the switch element 31a, and a voltage VQb at a connection point Qb between the auxiliary coil 32a and the switch element 31b change like a pulse with the reduction of the current of the auxiliary coils 32a and 32b by approximately xcex94I due to the electromagnetic induction. When the maximum change speed is 0.015A/ns and the current Ih of the magnetic field generating coil 30 is switched, the peak value Vp1 of the pulse-like voltage is about 15 V.
In the magnetic head drive circuit described in the above mentioned gazette, because it is not necessary for a magnetic head to generate a magnetic field when an information signal is not recorded, it is desired that the consumption of power be reduced by suppressing a current supply from the direct current sources 33a and 33b. To do this, it is necessary to set both switch elements 31a and 31b in the OFF state when an information signal is not recorded.
However, if an information signal is not recorded and both switch elements 31a and 31b are set OFF, then the current Ia of the auxiliary coil 32a and the current Ib of the auxiliary coil 32b are reduced from Is to 0. Conventionally, the operations have not been carefully considered when both elements 31a and 31b are set OFF, and the speed of the reduction of the current Ia and the current Ib is nearly equal to the speed of the change when the current Ih of the magnetic field generating coil 30 is switched during the process of recording an information signal. The voltage VQa at the connection point Qa and the voltage VQb at the connection point Qb temporarily increase with the reduction of the current Ia of the auxiliary coil 32a and the current Ib of the auxiliary coil 32b due to the electromagnetic induction.
Assuming that the inductance of the auxiliary coils 32a and 32b are L, and the change speed of a current is dIx/dt (where Ix is the current Ia of the auxiliary coil 32a or the current Ib of the auxiliary coil 32b), the voltages VQa and VQb substantially match xe2x88x92Lxc2x7dIx/dt. For example, assuming that L is 50 xcexcH, and |dIx/dt| is 0.015 A/ns at maximum, the peak value Vp2 of the voltage VQa and the voltage VQb is about 750 V. Since the voltages VQa and VQb are applied to the switch elements 31a and 31b in the OFF state, it is necessary to set the resistible voltage Vt higher than the peak value Vp2 of the voltages VQa and VQb (for example, at 800 V) to prevent the destruction of the switch elements 31a and 31b. 
At this time, for example, assuming that a MOS FET (MOS type electric field effect transistor) is used as a switch element, the ON resistance (between a drain and a source in the ON state) normally increases with a higher resistible voltage between the drain and the source. Therefore, in the above mentioned magnetic head drive circuit, the ON resistance cannot be reduced because of the restriction of the lower limit of the resistible voltage Vt of the switch element. That problem causes power consumption to increase. In addition, to raise the frequency of an information signal to be recorded, it is necessary to switch the current Ih of the magnetic field generating coil 30 at a higher speed. However, at this time, the speed of reducing the current Ia of the auxiliary coil 32a or the current Ib of the auxiliary coil 32b is also reduced when the recording process is stopped, and the peak value Vp2 of the voltages VQa and VQb rises. Therefore, a higher switch element has to be used for the resistible voltage Vt. However, since the resistible voltage Vt of a practical MOS FET is about 1,00 V at most, there has been the problem that the frequency of an information signal cannot be raised at a higher recording speed.
In addition, assuming that a bipolar transistor is used as a switch element, the gain band width product fT is normally lowered as the resistible voltage between a collector and an emitter becomes high. Therefore, in the magnetic head drive circuit, the gain band width product fT cannot be raised by the restriction of the lower limit of the resistible voltage Vt of the switch element. That causes the problem that the frequency of an information signal to be recorded cannot be raised at a higher recording speed.
The present invention has been developed to solve the problems of the above mentioned conventional technology, and aims at providing a magnetic head drive circuit and a magneto-optical recording device capable of reducing the voltage of an auxiliary coil generated by the electromagnetic induction when the process of recording an information signal is stopped, reducing the consumption of electric power, and raising the recording speed.
The above mentioned objects of the present invention can be attained by the following configuration.
A magnetic head drive circuit for applying an electric current to a magnetic field generating coil of a magnetic head for recording an information signal includes:
a pair of auxiliary coils;
a pair of switch elements connected in series to the pair of auxiliary coils; and
a switch element control circuit for controlling the switch elements,
wherein the magnetic field generating coil is provided between the connection points of the pair of auxiliary coils and the pair of the switch elements, the switch element control circuit switches the direction of the drive current flowing through the magnetic field generating coil by alternately setting ON the pair of the switch elements according to an information signal during the recording of an information signal, and holds the switch element in the ON state when the recording of the information signal is stopped, and then switches the switch element from the ON state to the OFF state slower than in the recording of the information signal.
The object of the present invention can also be attained by the following configuration.
A magneto-optical recording device includes:
the above mentioned magnetic head drive circuit, and an optical head,
wherein a magnetic field generated according to an information signal to be recorded is applied from the magnetic field generating coil to a magneto-optical recording medium, and the optical head emits an optical beam to a portion to which the magnetic field is applied, thereby recording an information signal to the magneto-optical recording medium.
This is described later in detail.